How milk makes a frothy coffee, or not

James - As I’m sure many are aware, we go to the effort of frothing milk in some coffees to give it a creamier texture to the drink. This foam is often thought to be due to the protein in the milk interacting with the coffee and air. Paul on the forum explains, saying ‘Milk is a complex mixture of many substances in addition to protein and some of these substances may act as surfactants, for example phosphatidyl choline and phosphatidyl inositol.’ and so heating up the milk and deforming those proteins might cause the froth to flop. But expert in nanocoatings, Professor Steven Abbott, is here to set the record straight:

Steven - Although everyone says that the foam is stabilised by denatured proteins, this isn’t true. For example, it is one of those “well-known facts” that milk containing just whey proteins (which don’t easily denature) foams differently from milk containing just casein (which is easier to denature), but look closely at the literature and you find that it’s not a fact at all. Different steam wand techniques produce different results, and some find, as I would expect, little difference in foamability between all-whey, all-casein or a standard mix. It’s proteins at a delicate balance between liking the water and liking the air that stabilise the foam. If you overheat milk with a steam wand you destroy all the protein and can’t make a foam. But it doesn’t take a lot of protein to stabilise the foam, so if you have, say, 50% of your protein remaining, you’ll be fine. If we want good foam, we should focus on getting lots of small bubbles. The recent success of the Nanofoamer (a clever whisk design) shows that you can produce a great foam even with cold milk (dairy or non-dairy), and with no denaturing of the protein.